In the field of heat treating, objects to be treated are subjected to extremely high temperatures ranging from about 500.degree. C. to about 1500.degree. C. for varying time durations. The machinery used to apply the desired heat treatment must also be able to withstand extreme heat conditions of the process. Metals were the first materials available for such applications. However, even the most durable metals and metal alloys suffered from structural fatigue due in part to oxidation and carburization. Curved pipe segments, i.e. pipe segments having a curved centerline, also suffered from the additional problem of being more flexible and weaker than the straight pipe segments.
Alternative materials were investigated as they became available. Ceramics and other refractory materials were incorporated into components for heat-treating systems. Layers of monolithic refractory materials were first applied to steel piping used in heat-treating systems. While such ceramic layers imparted additional strength (and reduced flexibility) to the U-bends, various manufacturing constraints kept such materials from being commercially successful. Such ceramic-lined parts still began to fail when exposed to temperatures in excess of 1000.degree. C. for the necessary prolonged periods of heat-treating processes.
Further it was expensive to manufacture such lined pipes. The steel layers being coated had to be pretreated or presented with barb-like anchors or filaments to retain the ceramic layer which increased the overall manufacturing cost. In addition, shrinkage problems were experienced as the ceramic material dried. Such shrinkage problems led to inconsistent layering characteristics at best, and outright material failure in the form of the ceramic layer cracking, peeling, etc. at worst.
In addition, during heat-treating applications piping with internal ceramic coatings, especially at bends, would necessarily experience accentuated effects of material expansion and contraction. Therefore the material used to make the bends would need to be resilient and behave flexibly without being too flexible.
Better methods of casting and extruding ceramic piping were developed, and straight segments of durable and acceptable ceramic piping were achieved. However, repeated attempts to manufacture the required curved segments of ceramic pipe for use in the high temperature heat-treating systems met with failure. Particular difficulty came in manufacturing "U-bends". U-bends are segments of piping which are used to redirect, in one segment, the flow contained within a piping system by 180 degrees, or a direction close to 180 degrees. The overall shape of the pipe segment is therefore that of a "U". Acceptable ceramic U-bends useful in heat-treating systems could not be made according to conventional molding systems due largely to initial manufacturing problems, such as material shrinkage, as the ceramic dried. Without useful bend segments, the widespread use of linear ceramic piping in the heat-treating field remains stymied.
The success and usefulness of a ceramic heat-treating system depends upon the entire system being able to uniformly withstand certain temperatures for prolonged periods. The inability to successfully develop reliable ceramic U-bend pieces has hampered the effectiveness of ceramic pipe heat-treating systems. A method of reliably casting ceramic U-bend segments able to withstand the severe temperature conditions of heat-treating systems would be highly advantageous.